1. Field of the Invention
The present invention relates to a mobile station that selects one of a plurality of base stations according to the electric field strengths of control channels of the plurality of base stations and establishes a link and a call connection with the selected base station on the traffic channel assigned by the base station. The present invention also relates to a base station for the mobile station.
2. Background Art
In recent years, mobile stations for use in mobile communication systems, such as PHS (Personal Handy phone System) and PDC (Personal Digital Cellular telecommunication system), have been brought into common use because of their reduced size and weight and the improved communication quality.
In the PHS and PDC, communication takes place between mobile stations and base stations using traffic channels (hereinafter abbreviated to “TCHs”) and control channels (hereinafter abbreviated to “CCHs”). Here, a TCH means a channel used for the communication between a mobile station and a base station and a CCH means a channel used to control the switching to a TCH when a mobile station places or receives a call in a standby status.
FIG. 7 shows base station selection processing performed by a conventional mobile station (referred to as “PS” in the drawing) based on a PHS method, and a link channel establishment sequence performed when a CCH between the mobile station and a conventional base station (referred to as “CS” in the drawing) is switched to a TCH.
The base station selection processing is performed, for instance, when the mobile station is (1) switched on, (2) searching for a new base station during handover, and (3) designated by a current base station to switch to a new base station during communication.
During the base station selection processing, the mobile station first monitors CCHs of a plurality of base stations. In the case of the PHS, a specific frequency is allocated to CCHs and the plurality of base stations transmit signals on their CCHs by time division multiplexing. The mobile station sequentially receives a signal transmitted on each CCH, decodes the signal, and, if the identifier of a base station (CSID) is detected from the decoded signal, stores the CSID along with the electric field strength of the signal (S51). In this manner, the mobile station obtains a plurality of pairs of CSIDs and electric field strengths. The mobile station sorts the pairs in decreasing order of electric field strength (S52), and selects a base station having a CSID that is paired with the highest electric field strength (S53). The mobile station then enters into a standby status on the CCH of the selected base station. If the base station selection processing is performed immediately after the mobile station is switched on, the mobile station performs location registration for the selected base station before entering into the standby status.
The link channel establishment sequence is performed, for instance, when the mobile station (1) places a call in a standby status, (2) receives a call in a standby status, and (3) switches to a new base station during handover.
In the link channel establishment sequence, the mobile station issues a link channel establishment request to the base station, which has been selected in the base station selection processing, on the CCH of the base station (S151), receives a link channel assignment message from the base station (the base station issues the link channel assignment message in response to the link channel establishment request) (S152), and measures the electric field strength of the U wave (undesired wave) of the assigned TCH (the TCH specified by the link channel assignment message) (S153). Because this measurement is performed immediately after a link channel is assigned, which is to say before any signal is exchanged between the base station and the mobile station on the TCH, the measured electric field strength represents the level (electric field strength) of the U wave (an interference wave) of the assigned TCH. If the level of the U wave is high, the processing returns to S151 and the stated operations in S151-S153 are performed again.
If the level of the U wave is low, the mobile station then transmits synch burst signals to the base station (S154), the base station returns the synch burst signals to the mobile station (S155), the mobile station transmits TCH idle burst signals to the base station (S156), and the base station returns the TCH idle burst signals to the mobile station (S157). In this manner, synchronization is established on the TCH. The stated processing until the synchronization establishment is the link channel establishment sequence and corresponds to Layer 1 (the physical layer).
After the link channel establishment sequence, the mobile station checks whether the base station is ready for communication and then the base station checks whether the mobile station is ready for communication (S159 and S160). This processing corresponds to Layer 2 (the data link layer).
Following Layers 1 and 2, call connection messages or location registration messages are exchanged between the mobile station and the base station. This processing corresponds to Layer 3 (the network layer) and above.
The base station selection processing and the link channel establishment sequence for switching from a CCH to a TCH have been described above by taking the case of the PHS as an example. However, similar processing is performed in other systems, such as PDC, where channels are classified into CCHs and TCHs.
In some cases, the mobile station is assigned by the base station a TCH whose electric field strength is extremely lower than that of the monitored CCH and is insufficient for actual communication, such as conversation. In such cases, there are various problems, such as a high frame error rate, inappropriate message exchanges in Layers 2 and 3, and a wireless connection failure.
This is due to the transmission power control by the base station and the change in reception condition of the mobile station. The transmission power control means the adjustment of transmission power by the base station according to the electric field strength of a transmission signal from the mobile station. In many cases, a base station in the PHS uses a power of 500 mW for transmission on a CCH and uses a power ranging from 20 mW to 500 mW for transmission on a TCH by performing the transmission power control. As a result, in some cases, the electric field strength of a TCH assigned to a mobile station significantly differs from that of a CCH. Also, the reception condition of a mobile station changes considerably due to the change in direction and location of the mobile station and the influence of nearby buildings (the location of the mobile station in particular changes while the user is moving at high speed). As a result, in some cases, a mobile station is assigned a TCH whose electric field strength is insufficient for actual communication.